The invention relates to unidirectional AC/AC and DC/AC converters and a bi-directional DC/AC converter. This includes switching power amplifier, AC power source, frequency converter, line conditioner and uninterruptible power source.
Many terms exist to describe various types of devices used for power conversion. The following definitions are provided in order to avoid any conflict of terms. A switching power supply (SPS) is an AC/DC or DC/DC converter. A switching power amplifier (SPA) is an AC/AC or DC/AC converter. An SPA that produces a fixed frequency is commonly referred to as inverter, AC voltage regulator, AC power source, line conditioner, frequency converter, etc. An SPA that amplifies a variable frequency is often narrowed to class-D amplifier, whereas other techniques exist. An uninterruptible power source/supply/system (UPS) is a bi-directional DC/AC converter in which energy is delivered from a rechargeable battery to an output, and vice versa. The UPS charges the battery when line is present and simulates line voltage when line fails. However, a low-cost UPS usually produces a square wave voltage that has adequate RMS level. In the following disclosure, the term converter refers to a block performing an essential function within a parent apparatus.
Conventional SPA and UPS each comprise an output inductor coupled in series with an output capacitor. A positive supply voltage selectively applied to the inductor is greater than positive peak of the output voltage. Similarly, a negative supply voltage is smaller than negative peak of the output voltage. In the SPA, a pair of ungrounded switches drives the inductor. One switch is referenced to the negative supply voltage, whereas the other switch is floating. In the UPS, a DC/DC converter and a battery charger are added to the SPA. A single battery voltage is converted into the supply voltages of the SPA. If a transformer is used but line isolation is unnecessary, one of the output switches is grounded. The other switch remains floating. During the battery charging, the switches used in the DC/DC converter and the SPA are idle. The battery charger employs an additional switch that is used exclusively for charging. An ordinary battery is tolerant of charging current waveform. Therefore, low cost of the charger is the primary concern. The charger acts like an SPS. Conventional SPSs commonly use power factor correction to produce a sinusoidal input current. By contrast, a sinusoidal current for charging the battery is unheard-of in low-cost UPSs.
The present invention is intended to provide low-cost SPA and UPS with sinusoidal output. For example, the SPA or UPS can be built using only six power components. These include a battery, a pair of MOSFETs with on-chip diodes and a capacitor providing the output voltage. Moreover, line isolation is accomplished. Therefore, a procedure of replacing the battery is safe and isolated from electrical hazards. The UPS requires a relay and a diode bridge to produce a pure sinusoidal current for charging the battery. This double-pole double-throw relay incorporates a transfer switch.
The SPA and UPS are intended to provide an AC output voltage in response to a reference voltage. However, it is necessary that an output current have a negligible DC offset. Accordingly, the load is preferably symmetrical, i.e. capable of drawing the same current independently of polarity. One exemplary application is a UPS driving a typical SPS. The SPS presents a load that is equivalent to a diode bridge driving a large capacitor and a parallel-coupled resistor. If the SPS is power factor corrected, the load is equivalent to a diode bridge driving a resistor. In either case, if one of the diodes in the bridge fails to conduct, the SPS continues to operate properly. However, this puts disproportionate stress on undamaged conducting diodes, protection devices, the capacitor, switches, etc. The SPS can malfunction and eventually fail completely. The UPS immediately detects this partial failure and prevents further destruction of the SPS. Another exemplary application is an audio SPA. An audio signal lacks any DC offset. Moreover, a loudspeaker represents a nonlinear load that may strongly depend on amplitude and frequency of the amplified audio signal, but remains symmetrical. An audio signal with DC offset or an asymmetric loudspeaker load indicates abnormal and possibly destructive condition. It should result in immediate shutdown of the SPA.
An asymmetric operation is acceptable, however. A first capacitor stores a DC voltage. A second series-coupled capacitor provides the output voltage. In order to maintain the DC voltage within a predetermined range, an average value of the output current must be zero. Accordingly, a current charging the first capacitor must be equal to the current discharging it. This is inherent if the output voltage lacks DC offset and the load is symmetric. Otherwise, an adequate DC offset has to be added to the output current in order to adjust its average value to zero. This can be accomplished in various ways since waveform of the output voltage is insignificant from the viewpoint of the circuit operation. For example, the output voltage can be clipped, amplified at polarity dependent gain or shifted by an adequate DC offset.
An example of an asymmetric load is a resistor in parallel with an equal resistor that is in series with a diode. The diode is cut off when the output voltage has one polarity. Otherwise, the diode conducts causing both resistors to evenly share the output current. The output voltage has to be effectively cut in half at latter polarity so that the average output current remains zero. If the load asymmetry is not counterbalanced, the SPA or UPS becomes inoperative. In another example, the load consists of only one resistor in series with the diode. No compensation is possible since the output current is unidirectional. A control circuit senses the DC voltage stored in the first capacitor. If the voltage becomes excessive, the SPA or UPS shuts down. In order to avoid that, a minimum load is added. The minimum load allows at least a minimal current flow in both directions.
An instantaneously interruptible power source (I2PS) is introduced in this disclosure and the disclosure of the co-pending application titled xe2x80x9cPrecision Switching Power Amplifier and Uninterruptible Power System,xe2x80x9d filed on even date herewith. A unidirectional or bi-directional I2PS is equivalent to a conventional SPA or UPS respectively. However, some intrinsic features of the I2PS are in sharp contrast to common flaws of the conventional devices. The I2PS can instantaneously interrupt the correction, wherein a precise correction can be accomplished in every switching cycle. Moreover, the I2PS can become idle by the end of every switching cycle or remain idle over a period of many cycles. No energy is wasted on useless, ineffective or bogus correction when correction is unnecessary. Usually, accuracy of the output voltage produced by the UPS is nonessential. However, the employment of the bi-directional I2PS, in place of a traditional UPS, results in reduced power dissipation. A less frequent correction of the output voltage is necessary. The I2PS is unidirectional, unless otherwise noted.
The unidirectional or bi-directional switching power apparatus according to the present invention converts at least one supply voltage into an AC output voltage. In one embodiment, a converter means converts the supply voltage or voltages into a current, and includes an inductive means for attaining the current. A first capacitive means stores a DC voltage. A switching means selectively applies the current to the first capacitive means. A rectifying means rectifies and applies the current to the first capacitive means. A second capacitive means is coupled in series with the first capacitive means for providing the AC output voltage. In another embodiment, a converter means converts the supply voltage or voltages into a binary voltage. An inductive means provides a return voltage in response to the binary voltage. A rectifying means limits the return voltage. A first capacitive means stores a DC voltage. A switching means selectively applies a sum of the DC voltage and the AC output voltage to the inductive means. A second capacitive means is coupled in series with the first capacitive means for providing the AC output voltage. In yet another embodiment, a source means provides the supply voltage or voltages. A first capacitive means stores a DC voltage. A first converter means converts the supply voltage or voltages into a first current applied to the first capacitive means. A second converter means converts a sum of the DC voltage and the AC output voltage into a second current applied to the source means. A second capacitive means is coupled in series with the first capacitive means for providing the AC output voltage.
A corrective current is equal to at least a portion of the current attained by the inductive means. The corrective current can be interrupted or uninterrupted even when the inductive means is connected to the first capacitive means. For example, the inductive means may consist of a flyback transformer. This transformer provides a secondary current when a primary current is interrupted, and vice versa. The corrective current is inherently interrupted in unidirectional and bi-directional I2PSs. The first or second switching means can instantaneously interrupt flow of the corrective current. A current flowing through the second capacitive means is equal to a difference between the corrective current and the output current of the I2PS. The latter current may be zero since no minimum load is required. Any UPS or bi-directional I2PS is capable of charging the battery when a low frequency voltage, in particular line voltage, is applied across the second capacitive means. Preferably, the charging is carried out at both halves of the AC voltage so that an average value of the charging current drawn from the AC source is zero. Moreover, the UPS or bi-directional I2PS can produce quasi- or pure-sinusoidal charging current. Obviously, any UPS or bi-directional I2PS can operate as an SPA or a unidirectional I2PS respectively. A conventional power supply can be substituted for the battery.